"Silicones" are synthetic polymeric materials that possess an extraordinarily wide range of physical properties. They can be low- or high-viscosity liquids, solid resins, or vulcanizable gums. They display an unusual combination of organic and inorganic chemical properties that are due to their unique molecular structure of alternating silicon and oxygen atoms; this "polysiloxane" chemical structure is common to all silicones. Silicone polymers can be mixed with other chemicals and fillers into an enormous variety of products that serve in a multitude of applications. For a general discussion of silicone chemistry see "Silicones", Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., 20, 922-962(1982).
The fundamental component of a silicone composition is the polysiloxane referred to earlier and as depicted below in formula F1. ##STR1## These polymers are made by an equilibrium process from other siloxanes and typically range in viscosity from about 0.01 Pa s to 2500 Pa s.
The room-temperature-vulcanizing ("RTV") silicones are a special class of silicones that have as a common attribute the development of a "crosslinked" elastomer from relatively low molecular weight linear polymers by means of a chemical reaction that forms these crosslinks and effectively extends chain length simultaneously. RTV silicones (e.g., addition cure silicones) have many applications in industry including use as sealants, caulks, adhesives, coatings, molding materials, dental impression materials and medical and dental implants.
An essential ingredient in an RTV silicone is a crosslinking component (hereinafter the "crosslinker") that reacts with the functional group or groups of the polymer chains (e.g., R.sup.1 and R.sup.2 of formula F1) to simultaneously lengthen them and connect them laterally to form the crosslinked network characteristic of a RTV silicone elastomer. Usually a catalytic agent is included to facilitate the reaction of the crosslinker with the polymer's functional groups.
There are many types of RTV silicones and likewise many types of crosslinking components and catalysts. Two such systems include (i) condensation cured silicones and (ii) addition cured, e.g., hydrosilylation cured (alternatively spelled "hydrosilation"), silicones. Condensation cured silicones characteristically, and in many instances detrimentally, release water (or alcohol) as a by-product of the crosslinking reaction. In contrast, there is no by-product in an addition cured silicone. The crosslinking reaction in these systems is typically triggered by combining the silicone polymer, the crosslinker and the catalyst. A variety of catalysts initiate and accelerate condensation curing such as amines and carboxylic acid salts of tin. At low temperatures the condensation cured silicone typically requires long times to fully cure (hours or even days). Higher catalyst concentrations and/or higher temperatures can shorten the cure time.
Unfortunately, condensation cured silicones typically suffer from an unacceptably large dimensional change upon curing or post curing. Applications which require precise dimensional accuracy (e.g., a dental impression) are adversely affected by these dimensional changes. When a condensation cured impression is used as a model for the formation of a dental crown or bridge the inaccuracy of the silicone is transferred to the dental crown or bridge. This results in a poorly fitting dental appliance which may cause pain or discomfort to the patient.
Addition cured silicones (e.g., hydrosilylation cured silicones) are generally considered to be of higher quality and are dimensionally more accurate than condensation cured silicones. Unlike condensation cured silicones, addition cured silicones do not produce detrimental by-products during curing. Addition cured silicones differ from condensation cured silicones in that the addition cured composition typically contains:
(1) a polymer which contains one or more vinyl functional groups, PA1 (2) a crosslinker component containing one or more Si-H bonds, and PA1 (3) a platinum catalyst. PA1 (a) curable silicone polymer, e.g., vinyl-containing organopolysiloxane, PA1 (b) crosslinker, e.g., organohydrogenpolysiloxane, PA1 (c) platinum catalyst of the Karstedt type and PA1 (d) stabilizer, e.g., amine stabilizer. PA1 (a) organohydrogensilanes having the empirical formula, EQU (H).sub.a (R.sup.3).sub.b Si.sub.c (F 2) PA1 (b) organohydrogencyclopolysiloxanes having the empirical formula, EQU H.sub.d R.sup.3.sub.e (SiO).sub.f (F 3) PA1 (c) organohydrogenpolysiloxane polymers or copolymers having the empirical formula, EQU (H).sub.g (R.sup.3).sub.h Si.sub.j O.sub.(j-1) (F 4) PA1 (a) unsaturated silanes having the empirical formula, EQU R.sub.a R'.sub.b Si.sub.c X.sub.z, (F6) PA1 (b) unsaturated linear or branched siloxanes of the formula, EQU R.sub.d R'.sub.e Si.sub.f O.sub.(f-1), (F7) PA1 (c) unsaturated cyclic siloxanes of the formula, EQU R.sub.d R'.sub.e Si.sub.f O.sub.f, (F8)
A particularly preferred addition cured silicone is formed by reacting (1) a vinyl-containing organopolysiloxane with (2) an organohydrogenpolysiloxane. This reaction is typically facilitated by the presence of (3) a platinum catalyst of the Karstedt type. Platinum catalysts of the Karstedt type are described in U.S. Pat. Nos. 3,715,334, 3,775,452 and 3,814,730 which are herein incorporated by reference.
When RTV silicones are used as modeling compounds (e.g., dental impression materials) it is customary to provide the compound to the user as two separate mixtures (i.e., the catalyst is separately stored from the crosslinker). When the user is ready to prepare an impression or model she will mix the two parts together, place the silicone against the surface or object to be modeled and then wait until the silicone completely cures. The cured silicone is then removed from the surface or object and retains a negative impression of that surface. A positive model may then be formed by filling the impression cavity with a material such as gypsum or plaster of paris. In many instances it may not be feasible to form the positive model immediately. Therefore, it is also important that the impression retains its dimensional accuracy over a long period of time (often weeks or months).
The setting reaction of an RTV silicone is triggered, in general, by the mixing together of the catalyst, crosslinker and polymer. By varying the amount of catalyst and crosslinker, the rate of setting may be adjusted. As the material begins to set its viscosity increases. Eventually, the mixture becomes "gelled" and is irreversibly changed into a crosslinked polymer or an "elastomer." At the gel-point the material no longer easily flows or adapts to new shapes. Therefore, in applications such as dental impressioning this period of time defines the extent of the "working time" period.
When the reaction is complete the material is said to be "set." This "setting time" is likewise an important parameter for a silicone impression material as it is crucial that the material remain against the surface it is to replicate until it has completely set. It is desirable to have a short setting time (e.g., less than 10 minutes). Premature removal from the surface being replicated may result in a distorted impression which will continue to crosslink, in the distorted position, outside of the mouth. Unfortunately, this situation is often initially unnoticed by the dentist and is discovered only after an expensive, but worthless, dental appliance has been fabricated. The dentist and patient must then go through the whole lengthy process again. This is a great expense and inconvenience.
The rate of setting may be further adjusted by the incorporation of well known inhibitors and/or retarders. One such inhibitor is 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane. These retarders often operate by competitively reacting with the catalyst thereby slowing the crosslinking reaction. In general, with the slowing of the reaction both the working time and the setting time are affected.
For applications requiring detailed reproduction, such as dental impression materials, the setting time and the working time parameters are very important and must be carefully controlled. As previously mentioned, the working time measures the time period over which the mixed silicone material remains fluid enough to flow and change shape. After the reaction has reached the gel point the material's properties change drastically and resist further fluid flow. It is desirable to have sufficient working time so that the dentist may easily, and prior to crosslinking, (1) mix the materials and (2) place them in the mouth.
One major factor which affects both the working time and the setting time is the catalyst activity. Unfortunately, platinum catalysts of the Karsted variety are somewhat sensitive to degradation and therefore are of variable activity. While the exact mechanism is presently unknown, this degradation may be advanced at high temperatures (such as might be encountered in a hot warehouse or in a truck-trailer). Over time the catalyst composition is believed to degrade and the setting time of the mixed composition becomes longer and longer. As previously mentioned even small changes in the setting time can have a detrimental affect on the accuracy of an impression if the user removes the material prior to its complete cure. Such early removal becomes more likely if the catalyst activity unexpectedly decreases upon storage. In extreme cases the silicone composition may never completely set due to this degradation effect. It would therefore be desirable to have a high-temperature storage-stable RTV silicone material which resists degradation at elevated temperatures.